A chain saw is essentially a power head that drives a drive shaft and provides for the mounting of a guide bar. The drive shaft is provided with a drive sprocket that meshes or mates with a saw chain which is mounted around the sprocket and guide bar. A loop of saw chain is essentially non-elastic and needs to be set to a precise degree of tension when entrained around the sprocket and bar. A chain that is too tight will cause power loss and rapid wearing, whereas a chain that is too loose will enable the chain to jump the track of the guide slot of the guide bar.
To permit mounting of the chain and subsequent adjustment of the chain tension, the bar is designed to be adjustable in a linear direction relative to the sprocket. Typically a pair of spaced apart mounting studs provided on the chain saw housing project through an elongated slot on the bar. The studs permit sliding adjustment of the bar toward and away from the sprocket but substantially only in a linear direction. (Some pivotal movement results from the tolerance between the slot width and diameter of the studs.) The studs are threaded and following adjustment, a clamping nut threaded onto one or both mounting studs secure the bar in place. Adjustment is typically achieved by a mechanism mounted on the chain saw housing. A nut is moved back and forth along the housing by turning a screw. A finger projected from the nut is inserted into a hole in the bar and turning of the screw imparts sliding movement of the bar along the studs.
The above typical arrangement is unsatisfactory for several reasons. The screw and nut adjustment mechanism add cost to the chain saw manufacture. Adjustment requires manipulation of a screw driver between the housing and bar. When mounting the bar and the chain saw, care must be taken to insure that the finger of the adjustment mechanism is inserted into the bar hole. The clamping nut is typically on the side of the bar opposite the adjustment mechanism and a novice user will occasionally try to adjust the bar without loosening the clamping nut. This can cause damage to the mechanism. There is also risk of injury to hands or arms from contact with the chain while reaching over the guide bar. Whereas installation of the chain has been described above, a chain will loosen over a period of operation and the tensioning process has to be repeated on a periodic basis.
It is accordingly an objective of the present invention to provide an adjustment mechanism within the bar itself, which is more readily accessible to the user, inexpensive to produce, improves safety, is easier to mount and is more reliable than the heretofore described typical mounting mechanism.